The radioelectric spectrum is a shared and limited resource. Therefore, each country establishes rules for the use thereof, specifying the spectral characteristics a radiofrequency transmission must have.
In digital communications systems, the information is encoded using symbols, to which a low-pass filter is applied to adjust the bandwidth of the signal to the width of the assigned channel. This filter, referred to as channel filter, must comply with the Nyquist Stability Criterion.
The most commonly used channel filtering system with digital modulated signals consists of two root raised cosine (RRC) filters, one in the transmitter and the complex conjugate thereof in the receiver. The combination of both filters results in an RC filter, which is characterized by reducing inter-symbol interference (ISI). This interference worsens the quality of the signal, making it difficult for the receiver to suitably decode the received symbols.
In order to assure interoperability between equipments from different manufacturers, communications standards, such as TETRA, define each and every one of the aspects of the various ISO levels. With respect to the physical level, they define characteristics such as the type of modulation, the data transmission rate or the RRC filter which must be used. The characteristics of said filter depend on two factors: the roll-off factor (α) and the symbol time (T). Specifically, the filter to be used by each of the modulations contemplated in the TETRA standard for voice plus data (V+D) systems is defined in document ETSI EN 300 392-2 “Terrestrial Trunked Radio (TETRA); Voice plus Data (V+D); Part 2: Air Interface (AI)”. For TETRA equipments in direct mode operation (DMO), the channel filter is defined in document ETSI EN 300 396-2 “Terrestrial Trunked Radio (TETRA); Technical requirements for Direct Mode Operation (DMO); Part 2: Radio aspects”. 
Document ETSI EN 300 392-2 defines all the air interface for TETRA systems supporting voice plus data (V+D) and it contains the specifications for the physical level, link level and network level, according to the ISO model.
The air interface for TETRA equipments in DMO, in addition to in the already mentioned ETSI EN 300 396-2, is defined in the following documents:                ETSI EN 300 396-3 “Terrestrial Trunked Radio (TETRA); Technical requirements for Direct Mode Operation (DMO); Part 3: Mobile Station to Mobile Station (MS-MS) Air Interface (AI) protocol”.         ETSI EN 300 396-4 “Terrestrial Trunked Radio (TETRA); Technical requirements for Direct Mode Operation (DMO); Part 4: Type 1 repeater air interface”.         ETSI EN 300 396-5 “Terrestrial Trunked Radio (TETRA); Technical requirements for Direct Mode Operation (DMO); Part 5: Gateway air interface”.         
All these standards define the various data units (PDU) used within layer 2, or link level, including the media access control (MAC) PDUs. The synchronization or SYNC PDUs are defined within this group of PDUs (ETSI EN 300 392-2 section 21.4.4.2; ETSI EN 300 396-3 section 9.1.1; ETSI EN 300 396-4 section 10.1.2; ETSI EN 300 396-5 section 14.1.1).
The SYNC PDU is transmitted either by the base station using the BSCH logical channel, or by a terminal in DMO mode in the SCH/S and SCH/H logical channels.
The “System Code” field is included among the various fields forming the SYNC PDU, which consists of 4 bits. The values comprised between 0000 and 0011 are used to indicate the edition of the EN/ETS 300 392-2 (“Terrestrial Trunked Radio (TETRA); Voice plus Data (V+D); Part 2: Air Interface (AI)”) and EN 300 392-7 (“Terrestrial Trunked Radio (TETRA); Voice plus Data (V+D); Part 7: Security”) standards; values from 0100 to 0111 are reserved for voice plus data (V+D) systems; 1000 and 1001 are reserved for generic use; and the remaining values correspond to the Direct Mode Operation (DMO) functionality, EN/ETS 300 396 standard.
Document ETSI EN 300 392-1 “Terrestrial Trunked Radio (TETRA); Voice plus Data (V+D); Part 1: General network design” defines in section 7.2.4 the composition of the identification number of a terminal or group of terminals, referred to as TSI (TETRA Subscriber Identity). The first field of the TSI is the MCC (Mobile Country Code), consisting of 10 bits and identifying in which country the TETRA system is located.
There are several documents in the state of the art which refer to methods for the variation of the radio parameters of a communications system which are similar to some of the features of the present invention, but which, when considered as a whole, basically differ from this proposal.
In this sense, reference could be made, for example, to document US2007005352, which describes a method for adjusting the channel filter to the bandwidth of the signal. It relates to a system used to perform the channel filter a filter bank with a bandwidth W, both in the transmitter and in the receiver, which allows grouping a number n of these filters to transmit and receive signals with a bandwidth n*W. It relates to a dynamic variation of the bandwidth of the signal in multiples of a predetermined one, W.
Document US2009036086 relates to a radiobroadcast system in which the bandwidth of the RRC filter of the transmitter is reduced to increase spectral efficiency, whereas the RRC filters of the receivers remain with a bandwidth that is greater than the former.
The objective of the present invention differs from the proposals of the earlier documents insofar as, as previously explained, it allows adjusting at least two radio parameters: the roll-off factor of the RRC filter and the transmission power level. Furthermore, the value of the configurable radio parameters is determined from over-the-air signaling contained in the SYNC PDU of the TETRA standard, in combination with the MCC (Mobile Country Code), which allows this configuration to be performed dynamically.
This allows:                Adapting this type of system to the specific radioelectric spectrum use requirements of each country.        Optimization of the radioelectric spectrum, without losing performance in data transmission speed and without the need to change the type of modulation.        Dynamic configuration of the radio parameters of all the equipments of the system, being of special interest for equipments which are registered for the first time in the system, thus preventing being able to perform transmissions outside the limits marked by the regulations for the use of the radioelectric spectrum in the country in which the system operates.        